In a hard disk drive, data is stored in a medium of magnetic material on the surface of the disk. The data is generally stored in concentric tracks, each track having a sequence of regions with opposing magnetic polarities. Thus, as one travels along the track, a change in polarity represents a "one," whereas the absence of a change in polarity represents a "zero."
To read the data from the disk, a magneto-resistive head is run along the track just above the surface of the disk. A change in polarity on the disk causes a change in the resistance of the read head. This change can be used to produce an output current from the read head circuitry. This is generally accomplished by driving two initially equal currents into the read head circuit. A change in the resistance of the head, which occurs when a flux change or "one" is detected on the disk, will result in the two output currents becoming unequal. A differential current output is therefore provided on two output lines from the read head. This differential current can be converted to a voltage, which in turn can be amplified and read by signal processing electronics.
A hard disk drive (HDD) unit generally contains several disks or platters in a column, each disk having a read head on either side of the disk. Generally, the output from all of the read heads will be channeled through one feedback and preamplifier circuit. Thus, only one read head is selected for reading by a switching system at any given time.
Each read head has slightly different magneto-resistive characteristics. A feedback circuit is used to correct for the individual characteristics of the selected read head. The feedback circuit senses an offset or differential current in the read head output when no flux change is detected (a "zero") on the disk. Since there should be no output when no flux change is being detected, the feedback circuit applies a small bias voltage to the read head. This bias voltage causes the output of the read head to become equalized. Thus, no differential current is produced as output by the read head unless a flux change is detected.
Problems arise when the switching system switches from one read head to another. Because magneto-resistive characteristics vary slightly from head to head, the bias voltage produced by the feedback circuit must likewise vary depending on which read head is selected. The design of the feedback circuit is such that a certain amount of time is required to fully respond to a change in the required compensating voltage. Thus, after a read head change, the output of the read channel will be under- or over-compensated until the feedback circuit adjusts, resulting in a temporarily incorrect output signal.
It is desirable to shorten the time required to adjust the read circuit feedback after a change in read heads. It is also desirable to do so without affecting the performance of the feedback circuit during normal operation.